feat(avatar): support guestFallback and version query params

[Antreesy]

Provide a full match the backend's getAvatar() method:

• guestFallback param (checked stable32 and newer)
• v param (added in perf(core): allow versioned avatar URLs to be cached for 30 days nextcloud/server#64295, not backported atm)

NcAvatar has almost identical implementation, so would make sense to use this one a single source of truth. vue-library version can be shortened to this (keeping dark-theme check as a missing gap):

export function getAvatarUrl(user: string, options?: AvatarUrlOptions): string {
      return generateAvatarUrl(user, {
              isDarkTheme: options?.isDarkTheme ?? checkIfDarkTheme(document.body),
              isGuestUser: options?.isGuest,
              size: options?.size,
              guestFallback: true,
              // version: options?.version,
      })
}

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.11%. Comparing base (6b5036c) to head (4369ba9).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #916      +/-   ##
==========================================
+ Coverage   93.58%   94.11%   +0.52%     
==========================================
  Files           2        2              
  Lines          78       85       +7     
  Branches       35       38       +3     
==========================================
+ Hits           73       80       +7     
  Misses          2        2              
  Partials        3        3              

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[Antreesy]

Rebased, only added test coverage (copied from nextcloud/vue)